Just thought: is it supposed to turn the entire bunch of trees into the cities, and surround the entire bunch of mountains with mines?
Viewing post in Gaia's Revenge jam comments
Yes. Sorry that this is not explained in-game at the moment.
Forests get turned into cities when humans manage to place 2 cities adjacent to them. So placing forest tiles with flanks exposed can quickly lead to a chain reaction in which whole groups of forests get flipped to cities in a single move. Forests are like a phalanx. They hold the line, but if they get flanked, it can quickly lead to a complete rout.
Mines and Farms each provide one extra move to the humans on their next turn, and can be placed within 2 tiles of another human tile. So a chain of mountains can rapidly be covered by mines, and a river with farms.
Perhaps I should nerf the humans' ability to place mines and farms so that mines and farms don't count as cities for the purpose of placing additional mines or farms.